The invention relates in general to valves and in particular to a new and useful metal-to-metal sealing mechanism for a vacuum valve, in which two concentric conically shaped spring elements cooperate with each other.
High vacuum valves with a metal-to-metal joint and cup-shaped stop members are known. In the prior art, preferably a relatively hard edge has been combined with a softer sealing mate. An example is shown in U.S. Pat. No. 3,108,780. With such a design, to ensure sealing after repeated shutoffs, the sealing force must slightly be increased upon every shutoff, i.e. the cup spring having the sealing edge on its periphery must recurrently be more and more expanded. This may be done up to a certain limit at which the softer sealing mate must be exchanged or, as disclosed in the above cited U.S. patent, the sealing edge must be displaced to reach a virgin sealing face portion.
High vacuum valves with a cup-shaped shutoff member such as disclosed in German OS Nos. 26 23 906 and 25 23 152, are an attempt to bypass this disadvantage by providing only relatively hard sealing mates and using a conical sealing seat at the same time as a stop. This design assumes that the cup spring, which is secured to an axially displaceable stem, becomes automatically centered by its rim in the conical sealing mate which, preferably, forms a part of the valve housing, and that in the course of further displacement of the stem with a contact on all sides and due to the expansion, the two sealing mates undergo a substantially elastic deformation, so that the sealing faces are not deformed permanently.
However, only with a highly accurate concentricity of the cup spring and the cone it may be expected that aside from a rolling motion, no further relative motion occurs on the sealing faces, particularly not parallel thereto, which would cause an alteration of the faces. In fact, already at the first shutoff operation, permanent deformations in the provided cooperating materials and a parallel relative motion of the sealing faces are to be taken into account. Primarily, the microgrooves resulting from the machining and other irregularities of surface texture are compensated for during the axial displacement, in a process of conformation.
This motion parallel to the sealing faces will cease only with the frictional forces exceeding the thrust. Only then starts the expansion of the cup spring leading to a further plastic deformation until the conformation of the surfaces reaches a degree at which the contact area is sufficiently large to absorb a further increase of the contact pressure elastically.
The expected mechanism of the sealing process at the rim of the cup spring in a prior art valve having a conical seat such as shown in FIG. 1, is illustrated in FIG. 2. FIG. 2 is a greatly enlarged partial view of the sealing zone in which cup spring 1 and conical surface 2 of FIG. 1 contact each other. In this illustration, it has been assumed that the cup spring and conical seat are made of high-grade, for example, stainless steel.
The dotted outlines 3 and 4 in FIG. 2 show the valve seat geometry in a valve which has already once been closed, at an instant shortly prior to the start of the expansion. The solid line contours 5 and 6 correspond to the state under full load. At 7 and 8, the vector of the closing force and the vector of the contact pressure at the circumference of the spring cup, respectively, are shown. The axial displacement during this process corresponds to the distance from point 9 to point 10. This displacement necessarily entails a damaging of the sealing faces, affecting their sealing function.
If harder sealing mates are selected, of the quality of a hardened ball-bearing steel, and an attempt is made to prevent the contact pressure action by a perfect surface geometry and mirror finish, the problem arises that soiling matter is only crushed on the surface and no longer embedded into the sealing material, whereby the sealing effect becomes problematic again.
The conditions are more favorable if the conical seat is made of a harder material, for example, stellite, and the spring cup of a relatively softer material, since then only the spring cup undergoes a plastic deformation. The manufacture of such a valve, however, is very expensive and even in this case, a sliding motion must be expected at least in some places at the circumference until the contact is established along the entire circumference, for example, if foreign matter is present on the surface, and also because a perfect roundness of the conical surface and the cup spring cannot be achieved.